1. Field of the Invention
The present invention relates to a height position detecting apparatus and a height position detecting method for detecting, by use of a laser beam, the height position of an upper surface of a workpiece held on holding means provided in a processing system.
2. Description of the Related Art
In semiconductor device manufacturing processes, a plurality of regions are demarcated by planned dividing lines called streets which are arranged in a grid pattern on a surface of a roughly circular disk-shaped semiconductor wafer, and devices such as ICs and LSIs are formed in the thus demarcated regions. Then, the semiconductor wafer is cut along the streets, whereby the regions with the devices formed therein are divided to produce the individual devices. Besides, an optical device wafer having a gallium nitride compound semiconductor and the like layered on a surface of a sapphire substrate is also cut along streets to divide the wafer into individual optical devices such as light emitting diodes and laser diodes, which are put to wide use in electronic apparatuses.
As a method for dividing the above-mentioned semiconductor wafer or optical device wafer or the like along the streets, a laser beam processing method has been tried in which a pulsed laser beam capable of being transmitted through the wafer is used to irradiate the wafer therewith in the condition where the converging point of the laser beam is adjusted to the inside of the region to be divided. In the dividing method using the laser beam processing method, the pulsed laser beam having such a wavelength as to be transmitted through the wafer, for example, a wavelength of 1064 nm is used to irradiate the wafer therewith from one side of the wafer while adjusting the converging point of the laser beam to the inside of the wafer, whereby an altered layer is continuously formed in the inside of the wafer along the streets, and an external force is exerted on the wafer along the streets where the strength of the wafer is lowered due to the formation of the altered layer, thereby dividing the wafer (see, for example, Japanese Patent No. 3408805).
However, a plate-like workpiece such as a semiconductor wafer has undulation, and, when the thickness of the workpiece varies, the altered layer cannot be formed uniformly at a predetermined depth position upon irradiation with the laser beam, due to a problem relating to refractive index. In order to form the altered layer uniformly at a predetermined depth position in the inside of the workpiece such as a semiconductor wafer, therefore, it is necessary to preliminarily detect the ruggedness of the region (street portions) to be irradiated with the laser beam, and to perform the laser beam processing while operating laser beam irradiation means in the manner of following up to the ruggedness.
Examples of proposals made for solving such a problem include the one described in Japanese Patent Laid-open No. 2007-152355. According to Japanese Patent Laid-open No. 2007-152355, height position detection means is used by which a surface of a workpiece held on a chuck table is irradiated with a visible laser beam and the height position of the surface (upper surface) of the workpiece is detected based on the amount of light corresponding to the area of reflection on the surface of the workpiece.
In the height position detection means described in Japanese Patent Laid-open No. 2007-152355, in the case where the wafer as workpiece is formed of silicon, the laser beam with a wavelength for detection (in Japanese Patent Laid-open No. 2007-152355, 635 nm) is not transmitted through the workpiece and, therefore, the amount of light corresponding to the reflection area on the upper surface of the workpiece can be measured accurately. In the case where the workpiece is formed of sapphire or quartz or the like which is transmissive to the laser beam with a wavelength of 635 nm, however, the laser beam for detection is transmitted through the workpiece and reflected also on the lower surface of the workpiece, making it impossible to measure only the light reflected on the upper surface of the workpiece. Therefore, by the detection system using the height position detection means disclosed in Japanese Patent Laid-open No. 2007-152355, it is impossible to detect the height position of the upper surface of a workpiece formed of a material which is transmissive to visible rays, such as sapphire and quartz.